This study was designed initially as a single site, Phase 1, double-blind, randomized, placebo-controlled, clinical trial in healthy adult volunteers to evaluate the safety and immunogenicity of a live attenuated ETEC vaccine, ACE527, with and without a mucosal adjuvant, dmLT. The addition of this challenge step provides a unique opportunity to evaluate the efficacy of the new lyophilized formulation of ACE527 vaccine, given in a two or three dose regimen, with and without dmLT, against wild type ETEC strain H10407 challenge (designated as Part B: Phase 2b Open Label Challenge Study). In addition, challenging the volunteers may allow for the identification of immune correlates of protection, taking advantage of newly available technologies (immune proteomics, transcriptomics, etc.) This is a first in man study to explore the immunogenicity and efficacy of lyophilized ACE527 alone and in combination with dmLT. The primary objectives of this study are to evaluate the safety, reactogenicity tolerability, and immunogenicity of a lyophilized, oral, live attenuated, ETEC vaccine designated ACE527 when administered orally with or without double mutant heat labile toxin (dmLT) at Days 0, 28, and 56; and to evaluate vaccine efficacy 4-6 months later after with ETEC H10407. The secondary objectives are to examine shedding of ACE527 during the week following each vaccination, to assess whether dmLT enhances the immunogenicity of the ACE527 vaccine, and to measure the protective efficacy of 2 or 3 doses of vaccine and vaccine plus adjuvant against severe diarrhea resulting from challenge with ETEC strain H10407.